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Modifications of sediment deliver by rivers, climate change and human impact are modifying the dynamics of beaches leading to departures from equilibrium and consequent beach evolution. When erosion occurs, concerns may arise for its impact on societal use of beaches and safety of human settlements and infrastructures. In these cases, beach preservation strategies allow us to mitigate the above impacts.
Groins are a widely used structure that is used to protect beaches from erosion. We define groins as shore-perpendicular structures aimed at either (1) maintaining the beach behind them, or (2) controlling the amount of sand moving alongshore. An example of beach protection with groins is shown in Figure 1.
Figure 1. Beach recovery strategy at Cogoleto (Liguria region, Italy; from APAT, Atlante delle Opere di Sistemazione Costiera, Manuali e Linee Guida, 2007)
Groins can be classified as either "long" or "short," depending on how far across the surf zone they extend. Groins that traverse the entire surf zone are considered "long," whereas those that extend only part way across the surf zone are considered "short." These terms are relative since the width of the surf zone varies with the prevailing wave height and beach slope. During periods of low waves, a groin might function as a "long" groin, whereas during storms it might be "short." Groins can also be classified as either "high" or "low," depending on how high their crest is relative to prevailing beach berm levels. "High" groins have crest elevations above the normal high-tide level and above the limit of wave runup on the beach (USACE, 1992).
Being directed perpendicularly to the shoreline, groins may not be effective if cross-shore sediment transport is dominant, such as is typical in shallow beaches. In fact, groins function best on beaches with a predominant longshore transport direction. They may also be inefficient if there is a large tidal range, allowing sand to bypass the structures at low tide or to overpass at high tide. Bypass of groins by sediments is defined as the movement of sediments along shore beyond the groins in the seaward direction. Overpassing is the movement of sediments above the groins when these are submerged.
Groins may be effective where sand movement alongshore is to be managed, such as where there is a divergent nodal region in longshore transport, for instance where the curvature of the coast changes, where intruding sand is to be managed, such as in correspondence of the banks of inlets, at the down-drift side of a large harbor breakwater, and other situations where longshore sediment transport is a reason of concern.
Groins may or may not be permeable to sediments. Impermeable groins are constructed of various materials, typically by boulders, and are designed to completely interrupt any available littoral drift in the proximity of the undisturbed shoreline. Thus, the littoral drift is forced around the seaward end of the groin or, if sufficient beach material has been trapped by the groin, to pass over its top. Permeable groins are designed so that an appreciable quantity of the available littoral drift will pass directly through its structural components. They are frequently built with wood elements.
Groin functional design has been discussed by several authors, including Bruun (1952, 1972), Balsillie and Berg (1972), Balsillie and Bruno (1972), Nayak (1976), Fleming (1990), and the U.S. Army Corps of Engineers (USACE, 1992). They offer the basis for empirical design based on laboratory experiments and field observations. The former may suffer from distorsion in small scale physical models while the latter suffer from specificity of beach conditions, wave forcing and sediment dynamics in general. A comprehensive discussion has been provided by Kraus et al. (1995).
Wave behaviours are leading parameters as they determine longshore sediment transport. Therefore, a rigorous statistical analysis of wave forcing, and in particular wave height and variability, is needed. Groin length is a leading parameter as well, and in particular the sead depth at the tip of the groin. Furthermore, another leading parameter for groin fields is the groin length to spacing ratio.
Practical experience reveals that modification of the shoreline accretion induced by groins rarely brings the shoreline itself to reaching the seaward end of groins. Typically, the updrift shoreline reaches only a modest distance to the groin tip. Such a behaviour indicates that sand bypassing and permeability of the groin to sand, as well as variability in wave forcing, play an important role in determining transport around the groin and resultant local and regional shoreline change. About the time of development of the new beach configuration, Nersesian et al. (1992) found that groin compartments were still slowly filling in the predominat direction of the shoreline sediment transport after almost three decades after displacement. Such functioning can be explained by the process of bypassing, whereby each compartment deprives sand to neighboring downdrift compartments during shoreline evolution.
Permeability is a desired feature of groins, in order to avoid accumulation of large volume of sand against them, which can be transported offshore during dominant cross-shore sediment transport. For the same reason, groins should not extend too far offshore beyond the average wave breaker line.
The evolution of the shoreline profile after the displacement of groins can be represented trough the progress in time of a suitable spatial coordinate x(t) which can be expressed through an analytical relationship. The variables that are important in groin design can be determined through dimensional analysis. In engineering and science, dimensional analysis allows one to inspect the relationships between different physical quantities by identifying their fundamental dimensions and units of measure and tracking these dimensions as calculations or comparisons are performed. We are not inspecting the details of dimensional analysis here. We assume that x(t) can be expressed in the form
where g indicates a variable or vector summarizing the groin features while w(t) and T(t) indicate the wave and tide forcing, respectively. Dimensional analysis for groin functioning indicate that several parameters may be effective to determine the shoreline evolution. The engineer can design some of them, like groin spacing, length and permeability, which is in turn determined by groin elevation and porosity. Other parameters related to wave forcing can be controlled by displacing other protection structures like breakwaters.
Kraus et al. (1994) provide a comprehensive overview of parameters governing the effect of groins. Parameters related to groin structure are:
Parameters related to beach and sediment are:
Parameters related to waves, wind and tide are:
Furthermore, functioning of groins may be affected by wave diffraction if it occurs. The impact of diffraction may be difficult to predict.
Design of groins can be carried out by applying empirical relationships derived from laboratory experiments and analysis of observations. The use of rules of thumb and empirical formulations is widely diffused as it is difficult to provide a physically-based interpretation of processes that are affected by large uncertainty. However, after a preliminary shape has been determined for groins it is suggested to carry out a numerical simulation of the beach evolution, which can provide support to the design and subsequent monitoring activities.
Preliminary design is often carried out by following the so-called “λ shoreline” rule of thumb, originally termed the “one-third rule” (Bodge 1998). The rule says that the post-project coastline during mean low water level is located between about λ=1/3 and λ=2/3 of the distance between adjacent groins measured behind the structures' seaward face (see Figure 2). Larger λ values, that may even exceed 2/3, may be appropriate for energetic environments, larger distances between groins, uncertainty in long term sediment supply and the whole design process. In general, larger λ values imply a more prudent design (Bodge, 2003). These values for λ are confirmed by several field observations that are summarized by Bodge (2003). The denomination “one-third rule” is explained by the above indicated minimum λ=1/3 value. The rule is considered applicable where the angle between the wave crest and the structures' control line is small, that is, lower than 25°-30°.
Figure 2. λ rule for the preliminary design of groins
Actually, the λ shoreline rule would predict a rectilinear line shape for the equilibrium profile of the beach when straight (non T-shaped) groins are displaced orthogonally to the shoreline. Actually, settlement of sediments tends to originate a curved shape of the after-project shoreline along the sides of the groin, for the effect of diffraction and sediment interception by groins. Empirical rules have been proposed to determine the actual equilibrium profile of the beach, which are not discussed here.
We adopt here the following steps for groin design as summarized by Bodge (2003).
Therefore one obtains
In practice, we select a number of beach cells, n, for a identified value of λ and desired value of X in order to develop a physically reasonable gap width, G. G values less than about 20 m are not generally recommended, at least for recreational beaches. In fact, small openings have limited aesthetic; moreover, strong rip currents may be originated with small openings.
On the other hand, gap widths greater than about 100 m are not recommended as well, because distant structures may be not effective in determining an extended reshaping of the beach.
Additionally, for recreational beaches, it is advisable that the head widths are smaller than the gap widths (H≤G).
Groin fields should be built from the downdrift to updrift direction. A desired evolution of groin field is that they are filled.
Main parameters for groin design are given in Figure 3.
Figure 3. Structure of a groin
Orders of magnitude for the principal parameters of groins are as follows.
Stability of groins should be verified against extreme sea conditions. If groins are made by boulders, the stability of the single stone should be checked against the expected wave energy.
The preliminary design of groins based on empirical rules should be verified through numerical modeling of beach evolution. Models are based on subdividing the shoreline in adjacent cells for which sediment transport rate is estimated by applying suitable relationship, like for instance the CERC formula. Limiting configurations of the beach, for instance due to emerging rock layers, are subsequently identified, as well as sources and sinks of sediments.
Once the model is set up, groins are displaced along the shoreline and modified sediment transport rates are computed. To this end, one need to estimate the volume of sediments that is intercepted by groins and the volume of sediments that bypass or overtop the structure.
Numerical models were proposed by the literature to estimate beach evolution after the displacement of groins. We summarise here below the theory of the GENESIS model (GENEralized model for SImulating Shoreline change; Hanson, 1988; Kraus et al., 1995). GENESIS simulates shoreline change as caused primarily by wave action. The model is based on the one-line concept, which assumes that the beach profile remains unchanged, thereby allowing beach change to be described uniquely in terms of the shoreline position. In fact, it is frequently observed that the beach profile maintains an average shape that is characteristic of that particular coast. Beach features like slope and singularities are preserved in the long term. Although seasonal changes in wave climate cause the position of the shoreline to move in a cyclical manner, the departure from the characteristic shape are relatively small. Therefore the beach profile responds to wave action by moving back and forth during erosion and accretion, under the assumption that the profile moves parallel to itself. If the profile shape does not change, any point on it is sufficient to specify the location of the entire profile with respect to a baseline. Groins may act as fixed points along the shoreline therefore inducing a seaward shift of the shoreline.
Important implications of the above assumption of steady profile are that only longshore sand transport can be taken into account and that the profile is always in equilibrium. However, cross-shore transport can be simulated within GENESIS in a schematic way, in terms of non-wave induced sources and/or sinks of sands along the coast.
The second major assumption of the model is that sand actively moves over the profile to a certain limiting depth, beyond which the bottom does not move. This depth is called the depth of closure Dc. GENESIS uses a simple relation for calculating the depth of closure according to the relationship
where Hmax is the maximum annual wave height. Evolution of beach occurs according the scheme presented in Figure 3.
Figure 3. Computation of beach profile under the effect of longshore sediment transport according to the one-line model. Source: tutorial of prof. Musumeci, University of Catania
The model is based on the application of mass conservation in each beach cell. For applications involving bypassing of sand at structures, knowledge of the depth to which sand is actively transported alongshore is required. This depth, assumed to be related to the incident wave conditions which vary with time, is called the depth of longshore transport. The latter parameter affect the rate of sand that is transported by waves around groins. A through analysis of groins bypassing should include the cross-shore distribution of the longshore sand transport rate, as well as the two-dimensional horizontal pattern of sand transport. In view of the uncertainties related to evaluating these processes, a simple assumption producing reasonable results was adopted.
The fraction F of sand that is transported over a groin and through it is represented by a permeability factor P, and the amount passing around the seaward end is represented by a bypassing factor B (Hanson & Kraus 1989), such that
where 0≤P≤1, and 0≤B≤1. P is estimated depending on the shape of groins and in particular the presence of apertures in their cross shore direction. P values are suggested by the literature for typical groin shapes. Groins made by boulders have a P value that is close to zero. The actual volumetric sediment transport rate at the groin Q'G is related to the calculated potential rate at the groin QG as
The permeability factor is assigned based on groin elevation, groin porosity, and tide range, and the bypassing factor B is calculated in the model at each time step through the relationship
where DG is the sea depth at the groin seaward end at a particular time step, and DLT is the depth of active longshore sediment transport, which can be assumed to be about 1.6 times the significant breaking wave height (Hanson & Kraus 1989). Therefore groin bypassing is dictated by sea depth at the groin seaward end and wave behaviors.
Empirical formulation are used within GENESIS to estimate bypassing for angled groins, shape of the sea bottom profile, impact of refraction and diffraction. We are not interested in the details herein. Once the after-project sediment transport rate is estimated, beach profile can be determined. The λ rule and other empirical rules of beach evolution can provide a benchmark to verify the reliability of the simulation. A user interface is available in GENESIS to allow implementation to a diverse variety involving arbitrary configuration and combinations of groins,
The stabilization of beach fill by structures may be warranted at sites where erosion stress is sufficiently severe to require otherwise impractical (frequent) renourishment intervals; or where the proximity of natural resources or marine structures preclude construction of a wide beach fill; or where the project shoreline is advanced far seaward of the adjacent shoreline. Adverse impacts to downdrift shorelines may be minimized by (i) advance-nourishment of the structures’ impoundment field with imported beach fill, (ii) use of T-head or other headland structures that do not promote rip currents and offshore losses, and (iii) termination of the structural field in non-accelerating longshore transport potential.
Beach fill stability is enhanced when the structures' heads are oriented such that the gaps between adjacent structures are approximately aligned with (or beyond) the angle of the local, incident wave crest. The mean low water shoreline within the beach cell between structures can be roughly estimated as lying a fraction λ of the distance of the cell's openings, behind a line drawn between the structures' seaward face, where 0.35
Particular care should be given to the preliminary interpretation of the processes leading to shoreline shape. Observation is the basis for a successful design.
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